Process for grinding and sifting

ABSTRACT

A process for grinding and sifting a granular material, more particularly cement, comprises grinding the charge material stream and thereupon sifting the ground material stream and dividing the latter into a coarse material stream and a fine material stream, and said course material stream into said charge material stream, a closed circuit of coarse material thus being formed, and said fine material stream forming the finished product. According to the invention, a partial stream is taken from said fine material stream and is reintroduced into said closed circuit.

Inventor Rudoll Ruegg Zurich, Switzerland Appl. No 805,752

Filed Mar. 10, 1969 Patented June 22, 1971 Assignee Escher Wyss Limited Zurich, Switzerland Priority Mar. 21, 1968, Dec. 24, 1968 Switzerland 4211168 and 19282168 PROCESS FOR GRINDING AND SlF'llNG References Cited UNITED STATES PATENTS Barthelmess Senseman Hardgrove... Wilson Hardinge Primary Examiner-Donald G. Kelly Attorney-Dodge & Ostmann 241/61 241/61 X 241/61X 241/19X ABSTRACT: A process for grinding and sifting a granular material, more particularly cement, comprises grinding the charge material stream and thereupon sifting the ground material stream and dividing the latter into a coarse material stream and a fine material stream, and said course material stream into said charge material stream, a closed circuit of coarse material thus being formed, and said fine material stream forming the finished product. According to the invention, a partial stream is taken from said fine material stream and is reintroduced into said closed circuit.

4 Claims, 6 Drawing Figs.

U.S. Cl 241/19, 241/29, 241/61 Int. Cl B02c 21/00 Field of Search 241/19, 29, 61, 24, 26, 30, 48, 97

PATENTED M22 I971.

SHEET 1 [1F 5 PATENTEUJUN22IB71 $586,248

sum 2 OF 5 Elsi I s 8 as PATENIH] m2 2 l9?! sum 3 [IF 5 PATENTEU JUH22 IQ?! SHEET 5 BF 5 PROCESS FOR GRINDING AND SIFTING BACKGROUND OF THE INVENTION Known plants for carrying out this process which comprise a mill and a sifter connected in the closed circuit on the coarsematerial side of said unit are designed for making always a single sort of cement. The known regulating systems for adhering to a prescribed degree of fineness are expensive and/or complicated in operation. Changeover of a plant designed for a certain cement quality to the production of another sort of cement of a greater fineness is also extremely complicated and time-consuming, while operation under conditions not corresponding to the original design is uneconomical.

SUMMARY OF THE INVENTION It is the purpose of the invention to overcome these disadvantages, and the invention relates to a process by means of which fine regulation of the finished product may be effected in the simplest manner in plants which are operated for the production of a finished product of the fineness for which they are designed, and by means of which process it is furthermore possible to operate a plant also for the manufacture of a finished product of greater fineness than that for which it has been designed, and in which the changeover for this purpose may be effected rapidly and in the simplest manner. For this purpose, a process of the hereinbefore described kind is carried out according to the invention in that a partial stream is taken from the fine material stream, and is introduced into the closed circuit.

Investigations have shown that the fineness of the fine material coming from a sifting device, other conditions being the same, is substantially proportional to the fineness of the material to be sifted. The invention makes use of this knowledge and produces a greater fineness of the material to be sifted by increasing its fine material proportion by supplying fine material from the fine material stream leaving the sifter.

BRIEF DESCRIPTION OF THE DRAWING Embodiment examples of plants for carrying out the process according to the invention are represented in simplified form in the drawing, in which:

FIGS. 1 to 4 show four such different plants,

FIG. 5 shows an axial section through the top part of a particular design ofa sifter as contained in the FIGS. 1 to 4, and

FIG. 6 shows a portion ofa section on the line Vl-VI of FIG. 5, corresponding parts having the same reference in all figures.

DESCRIPTION OF PREFERRED EMBODIMENTS In the FIGS. 1 to 4 is to be seen a mill 1 and a sifter 2. From the sifter 2, a coarse material conduit 3 leads to the mill inlet 4, in which a raw material conduit 5 also opens. A bucket elevator 6 connects the mill outlet 7 to the inlet of the sifter 2. Thus, on the coarse material side, the sifter is connected in the closed circuit of the mill. The fine material conduit 8 opens into a continuously adjustable switch 9.

In the embodiment example shown in FIG. 1, a finished material conduit 10 leads to the outside of the plant from one outlet side of the switch 9. A partial stream conduit 11 leads from the other outlet side of the switch 9 to the base of the bucket elevator 6. By optionally selectable setting of the switch 9, any partial stream may be taken from the fine material stream coming from the shifter 2, and may be introduced into the closed circuit of the mill 1 directly in front of the sifter inlet, hence into the stream of material to be sifted, i.e., the ground material stream coming from the mill. Thus, the fineness of the material to be sifted is increased, whereby an increase in the fineness of the fine material or finished material is brought about. The devices necessary for this purpose are extremely simple and cheap, and operation of the regulation is extremely time-saving and simple.

The plant shown in FIG. 2 differs from that shown in FIG. I merely in that the partial stream conduit 12 does not lead to the base of the bucket elevator 6, but to the base of a second bucket elevator 66 conveying only the material of the partial stream to the sifter inlet. This modification is suitable above all for plants which are subsequently modified for carrying out the process according to the invention, and in which the branched partial stream is so large that the original bucket elevator 6 is unable to convey the additional quantity of material. It may, however, also be advantageous where the maximum partial stream is so large that stoppage of the bucket elevator 66, when the entire fine material stream is discharged through the finished product conduit 10, results in an important saving power.

In the plant according to FIG. 3, the partial stream conduit 13 does not lead to the base of the bucket elevator 6, like the partial stream conduit 11 in FIG. 1, but to the mill inlet 4. By means of the switch 9, therefore, a partial stream of the fine material stream coming from the sifter 2 may be introduced into the charge material stream composed of the raw material stream and the coarse material stream. In this case, the partial stream is introduced into the closed circuit of the mill immediately in front of the mill 1. This modification has the advantage that part of the fine material coming from the sifter 2 is again subjected to a grinding process. This is of advantage when an inadmissibly large quantity of oversize particles is present in the fine material. It still has, however, the disadvantage that the fine material returned to the grinding process eventually produces unwanted agglomeration and a buffer effect in the mill.

The plant according to FIG. 4 combines the advantages of the plants according to FIGS. 1 and 3 and obviates their disadvantages to a considerable extent. In it, the elements of both plants are combined. The switch 9 is followed on the finished material side by a second switch 99, from which branches off not only the finished material conduit 10, but also the partial stream conduit 11 leading to the base of the bucket elevator 6, while the partial stream conduit 13, leading to the mill inlet 4, branches off the switch 9 as in FIG. 3. This plant makes it possible to introduce into the closed circuit of the mill a partial stream directly in front of the mill 1 and directly in front of the sifter 2, respectively, according to the quality of the fine material leaving the sifter.

The process according to the invention has the slight disadvantage that the specific power consumption of the plant increases with the quantity of fine material introduced into the closed circuit of the mill. The process is therefor above all suitable for the case where a plant is used mainly for the production of a finished product for the fineness of which it is designed, and only for a short time for the production of a finished product of greater fineness, as well as for regulating the prescribed fineness of the finished product in those cases where a plant is used for the manufacture of a product of the fineness for which it has been designed.

The economy of the plant improved if the sifting degree of the sifter may be varied, the sifting degree being understood to means the ratio of inlet fineness to outlet fineness. To make this possible, a grinding and'sifting plant of the kind described is so constructed that the sifter has a device for varying the sifting degree. The device should be inexpensive and, on the other hand, should only have to cover a limited fineness range. Two particularly inexpensive devices for varying the sifting degree of a known circulating air sifter are described in the following.

The circulating air sifter 2 represented in FIGS. 5 and 6 has an inner scattering chamber 53, a fine material separating chamber 54, concentrically surrounding the said scattering chamber, a scattering plate 55 and, above the latter, a selector wheel 56 in the scattering chamber 53 and a fan wheel 57 above the selector wheel 56. Scattering plate, selector wheel and fan wheel have a common drive 58. The scattering chamber 53 and the fine material separating chamber 54 are connected together at the bottom by louvres 59 and at the top by a connecting channel 510. As simplest means of varying the sifting degree, radially movable slide elements 511 are now provided in the axial region between the selector wheel 56 and fan wheel 57, which slide elements in their end position on the sifter axis side constrict the passage cross section between the chambers occupied by the said wheels 56, 57. These slide elements 511 are secured to radial rods 512 which are guided radially movable in openings 513 in the outer housing 514 and guides 515 on the inner housing 516. Such radial slide elements in circulating air sifters are known per se and are described, for example, in Swiss Pat. Specification No. 277,091.

As further very simple steps for varying the sifting degree of the sifter, the selector vanes 517, 518, 519 of the selector wheel 56 have different radial dimensions. The number of selector vanes shown and their radial dimension is not essential, and is merely fixed by way of example, solely from points of view of clarity. By sliding the sliding elements 511 towards the sifter axis, in addition to the aforesaid constriction of the passage cross section between the chambers accommodating the wheels 56, 57 whereby in known manner an increase is produced in the velocity of the circulating air, the number of fully effective selector vanes is also increased. in the position of the sliding elements 511 shown, the selector vanes 517 and 518 are fully effective, in the outer radial end position of the sliding elements 511 only a the selector vanes 517 are fully effective, and-in the end position on the sifter axis side, all the selector vanes 517,518,519 are fully effective.

Sifters with the devices shown for varying the degree of fineness are inexpensive and give only a limited fineness range, while devices with a large fineness range are very expensive. The large fineness range sought is obtained in the present grinding and sifting plant by returning a partial stream of the fine material stream to the closed circuit, and not by means of an expensive sifter. lf, now, the proposed inexpensive sifter is used in the limited fineness range covered by it, and the return ofa partial stream of the fine material stream to the closed circuit is used only for the higher fineness range going beyond the fineness range of the sifter, this results in a better efficiency of the plant, since the specific power consumption increases with the quantity of the fine material introduced into the closed circuit.

Thus, for example, it is advantageous to cover a fineness range of from 2,300 to 3,500 Blaine with a proposed sifter, and to use the regulation by return of a partial stream of the fine material stream to the closed circuit only for a higher fitness range.

lclaim:

1. A process for grinding and sifting a granular material, which comprises grinding the charge material stream and thereupon sifting the ground material stream and dividing the latter into a coarse material stream and a fine material stream, and returning said course material stream into said charge material stream, a closed circuit of coarse material thus being formed, said fine material stream forming the finished product, in which process a partial stream is taken from said fine material stream and is reintroduced into said closed circuit.

2. The process defined in claim 1, in which said partial stream is introduced into said ground material stream.

3. The process defined in claim 1, in which said partial stream is introduced into said charge material stream.

4. The process defined in claim 1, in which said partial stream is divided into two partial streams, one of which is introduced into said ground material stream, and the other into said charge material stream. 

2. The process defined in claim 1, in which said partial stream is introduced into said ground material stream.
 3. The process defined in claim 1, in which said partial stream is introduced into said charge material stream.
 4. The process defined in claim 1, in which said partial stream is divided into two partial streams, one of which is introduced into said ground material stream, and the other into said charge material stream. 